Silver plating has superior characteristics in low contact resistivity, heat resistance and the like, and is widely utilized for electric/electronic components, such as various contacts, terminals, connectors or switches, (for example, see Patent Literature 1 (Japanese Patent Application Laid-Open No. 2001-3194)).
Recently, electric cars, plug-in hybrid cars and the like have become popular, and in association with this, battery chargers, such as battery chargers for household use or rapid battery chargers, also become popular. A terminal of a charging connector to connect a car and a battery charger has to endure connection and disconnection actions over several tens of thousands of times in addition to a use under high voltage and high electrical current.
Herein, tin-plated or reflow tin-plated materials on a copper substrate are often used for the terminals of electric and electronic components above, and if a surface of the material can be excellently silver-plated, it is believed that superior abrasion resistance and electrical conductivity can be added to a terminal.
However, it is extremely difficult to plate silver, which is noble metal, on tin, which is base metal, and displacement between tin and silver occurs due to a potential difference between tin and silver (diffused from each other), and peeling of silver plating or the like happens to occur. From those reasons, it is a current situation where a technology to laminate excellent silver plating on tin plating does not exist.
In this regard, for example, in Patent Literature 2 (Japanese Patent Application Laid-Open H8-176883), a manufacturing method for plated materials including steps to establish a Sn-plated layer at least on a portion of a base material surface made from copper or copper alloy, and to composite-plate one or more types out of Cu, In, Ag, Zn and Sb on the Sn-plated layer is disclosed.
However, the manufacturing method described in Patent Literature 2 aims at manufacturing of the Sn alloy-plated material, and it is characterized by forming a Sn alloy-plated layer containing 80% to 99% of Sn (provided a total amount of Cu, Zn and Sb in the plated layer is 10% or less) on at least a portion of the base material surface, by heating a composite plating obtained in the step above. The technique is to alloy tin and silver by heating, and poor adhesion between tin plating and silver plating is not a serious problem (in other words, this is not a technology to laminate excellent silver plating on tin plating).
In the meantime, when a base material (metal material) is directly silver-plated and the metal substrate makes direct contact with the silver-plated layer, the silver-plated layer happens to be embrittled in association with diffusion and reaction of atoms in the metal substrate with silver.